McCain said he had discussed Lieberman's retirement with him many times "in general terms."

"I just respect his decision," said McCain, who added that he thought it would be "selfish" to try to persuade his friend to run again.

While remarking that no one is irreplaceable in the Senate, McCain said Lieberman has "played a very unique role, independent and bipartisan...Nobody understands Middle East issues like Joe." Pointing to the Connecticut senator's "knowledge, background and influence on national security issues," he said "that will be what's hard to replace."

Reflecting on how Lieberman broke with the party that had placed him on the national ticket in 2000 to endorse McCain for president in 2008, McCain recalled, "The moment I'll treasure most in the many years of our relationship is when he came to a town hall meeting in New Hampshire and endorsed me."

McCain said he believes that Lieberman's support "was key to securing independents" and said that led to his New Hampshire primary win and the GOP nomination.

Acknowledging that Lieberman has paid a political price among Democrats by backing McCain, the Arizona Republican pointed out that it was "not enough to lose his chairmanship" and "not amongst independents." Democratic leaders had considered but then backed off stripping Lieberman of his seniority after the presidential election.

McCain, who beat back a tea party primary challenge himself last year, said of his friend's political prospects in Connecticut, "I think he had tough fight but a deep reservoir of good will."

Although McCain and Lieberman sharply disagreed on the repeal of the "don't ask, don't tell" policy, McCain said Lieberman's successful leadership on that issue can be judged a signature achievement, "I think so. From his standpoint, it has to be," he said.

While saying he is not sure what Lieberman will do after his term ends, "I'm confident because of his commitment to Israel and peace in the Middle East, he'll play a major role there."

With the Senate chamber different these days after notable deaths and retirements of long serving members, McCain said, "I think there's probably a short term lack of corporate knowledge in the ways of the Senate. But in the long run, the institution goes on."